Electronic phase bridge



Feb. 12, 1946. R. H. BROWN 2,394,892

ELECTRONIC PHASE BRIDGE Filed July 16', 1943 L-IEP a lzvm/r #2 RoeERr5K0 w/v INVENTOR lvpuwr ATTORNE the cathode of the triode.

'is also capable of Patented Feb. '12, 1946 uu-irao ism-Es.

- ELECTRONIC mass Eamon Robert Henry Brown, Emporium, Pa assignor toSylvaniaElectric Products Inc.,- Emporium, Pa.,

a corporation of Massac Application JulylG, 1943.8erial No. 494,923

I 8 Claims." (oi. 111-95) This invention relates,to metering devices forA. C. measurements, and-more particularly to methods and apparatus forthe exact determine tion of impedances, frequencies, and phasedinerences of electrical signals.

The invention is based on a known combination g,

of a special A'. C. bridge for determiningimpedlance with a phasesensitive galvanometer, sometimes called cosine galvanometer)! ,Thelatter instrument has been (Proc. Phys. $00., London, 46, 181; 1934).Ac.-

cording to Qosens, plate voltage is supplied to a triode (or pentode, oranother suitable amplifier tube) by a .full wave rectifier, in the formofpulsating DC. In a typical circuit-ofthis kind l6 as'shown in Fig. 1,the midpoint-5 ofthe seccndary 6 of atransformer I is connectedsto theplate 8 of the triode T1, audits twoends are coupled to the two cathodesof two rectifier tubes, orto 'Their'respective plate 9 and iii areconnected to v the cathode ll of the triode T1, with resistances (R1? inseries'between each rectifier anode and voltage on the grid 12, thethe-triode duringone half cycle, will-be identical with that during theother half cycle, it the plate impedances of the two rectifier diodesare equal and if the twoseriesresistors (R1) are equal. 0 Accordingly, asensitive galvanometer G oi sum- 3 cient inertia shunted between therectifier plates v will not be deflected so long as the A. C. suppliedto the rectifiers and to the triode plate is syxn metrical during thetwo half cycles.

Phase, difierences between the; voltage .applie tothe secondary of thetransformer and that-of a signal of the .same frequency fed to the inputterminals I, 2 of the triode over a delayingnetworl':can'thenbemeas'uredin various ways, e. a.

byadju'sting a phase shifterin the grid circuit off the triode so-as toobtain zero reading'o'i the vanometer. The, galvan'ometer reading" willbe zero for a phase difl'erence of 90 between plate Y Q and gridvoltagesand a maximuinfor zero phase 5' .difference. For phase.difierenceshavingany arbitrary value (notequal' to 9 or 90);betw'een-thetwo voltages,' the galvanometer reading is appropriatelyproportional ;to ,the' cosineofthe phasedifierence. This measuringdevice is there- 50.

fore usually referred'to as a cosine galvanometer. While the circuit maythus'be used as a cosine galvanomet'er" for measuring phasedifllerences'it I I yieldingmeasurementsof frequencies and impedances ofany kind if properlybridge. d I

described by C. R. Cosens l0 with the two resistors,

For constant D. C. plate current through 25 larly at the input of thetriode.

-"'I'he galvanometer between the plates of the rectifler tubes measuresthe time average of volti'n response to the pulsating currents flowingalternately through the two rectifierpaths in series with the platespace discharge gap of the triode.

Thus the cosine galvanometer" fulfills a function similar to that of anordinary bridge circuit,

and the present invention contemplates the use 01 the phase bridge(which phrase will be used iherein to indicate this combination oi atriode plies) for measuring impedance's'and frequencies by meansof aproperly arranged auxiliary circuit.

Such an auxiliary circuit has been developed in the form ,of a specialbridge in the triodednput circuit ofthe phase bridge. As will beexplained the cathodes of. double diode as ho 2o-in detail hereinbelow;this speeialbridge'which the purpose oLa'dJusting the input voltage soas toobtain serves primarily phase of the triode 5 age dropsdevelopedover the two series resistors rectiilers, and A. C. sup

for measuring impedances, in connection with the The interestingfeature, of this bridges (e. g., as described byrJ- phase bridge.

combination of R. 'Barnhart, instruments, vol. 14, page 89,

- 1941) isthat-it offers the means for measuring 'complex impedancesinserted in one branch merely by varying an ohmic resistance in anotherbranch. The combination-and the "method of using it for measuringpurposes as has been de-,

scribed in the prior art iustmentioned disregards anumber of' errorsfound to be inherent in the nature ,of the combination which may, inmany practical cases, amount to values of substantial magnitude, so asto reduce the device to a mere indicator;

It is, thereioraa principal objectof the invention to provide means.whereby the inherent inaccuraciesof a'phase bridge areeliminated. I

' Another principal object is to provide for chm--- inatingerrorsinth'efuse of a phase bridge which during eachhalf cycle, suchforqexample as the amplification constant of the detector tube, or itsplate resistance, or the plate resistance ofthe 7 According .to anotherobject, "means are provided to determinethe impedance of an unknownelement, or thefrequency of a given A; C. voltage,

by two readings of .a'single variable resistance arm in'a first bridge,which will result in zero galvanometer jreadings in, an averagingrectifier result from the variations of, tube characteristics Anotherprincipalobiect provides means ent as the disclosure proceeds.

-' period, the plate current tube T1 will flow back to determining theexact value, or the absolute value '(to a high-degree of accuracy) of animpedance,- or its phase angle, from two difierent values sov chosen ona single adjustable resistor in a bridge circuit as to obtain zero in anauxiliary circuit.

According to afurther object of the invention,

means'are provided whereby areversal of polarity of the input voltageinto a bridge circuit is utilized to eliminate the inaccuracy in theconventional use of a phase bridge.

for

galvanometer readings .a,so4,aoa

the phase diflerence between it; and the "eflective plate voltage Ev.

Ig=AEm8ifn d-I-BEmOOS 'cz-i-C' The expectation that the galvanoinetercurrent would be zero for some particular value or a is verified byexperiment. Equation 2 would seem to indicate that there might be morethan one value oi u for which this would be true, but this A feature ofthe invention relates'to the elim ination of errors in the reading of aphase bridge which result from the fact that current is taken from thebridge by the'detector or which result from the presenceof undesiredarmonics in the supply or test voltage.

Another feature relates to the elimination of errors in the reading of aphase bridge whichare .caused by the diflerencein'peak grid input.voltages during tworeadings taken with reversed input polarity.

Another feature relates to the provision of a' phase shifter between oneset Of terminals of the grid input bridge, whereby exact measurementsbecome possible without resorting to a reversal of terminals and to acorresponding resetting of x the bridge resistance. I

Additional objects and features will beappar- The invention will nowbedescribed inconnection with the drawing, in which:

Fig. 1 is a circuit diagram of the known phasesensitive galvanometerarrangement which is used in the method according to the invention.

has not been observed to be the case in the work actually carried onwith-this circuit. 4

The use of the phase bridge has been explained in the prior art on theassumption that the values oi A and C in the foregoing equations arestrictly zero. While it has been indicated in the prior art that thiscondition is not exactly fulfilled, apparently no attempt ha been madeto investigate the nature oi. the undesirable inaccuracies resultingfrom this fact, to make an es- Fig. 2 is .a schematic diagram of theknown grid input circuit bridge used with Fi 1.

Fig. 3 is a diagram showing improvement incorporated in the combinationor Figs. 1 and 2 according to the invention.

" Referring now to Fig. 1, when an alternating current is flowing in theprimary ity of the transformer 1 shown, it is evident that during onehall of the grid-controlled one end of the transformer between one ofthe pairs of electrodes in the twin-diode T2, it will flow to thebetween the other diode.

other end of the transformer pair of electrodes of the twin- Indeveloping atheory for the operation of this circuit the tubecharacteristics may not be considered as constants, since the platecurrent is continuously varying from zero to its peak value.

and during the other half cycle I Let them be represented byarbitraryfunctions of time, Mt), r(t) and 2(t) representing the voltageamplification of T1, the plate resistance of T1, and the plateresistance of T2, respectively. The galvanometer current Iga through thegalvanometer G may be expressed by,

Iga=Zn(AnE m sin cm-F-BnEgn COS curl-Cit) (1) In this equation, thevalues an are simply related -t0'the phase differences mm between inputand various harmon eflective" plate voltage of the ics; Em is thevoltage amplitude of the nth harmonic fed to the grid input, and An,Bqand Cn,

are functions determineduby the circuit elements,

the construction of the tubes, and the range and manner of the platecurrent variation during each half-cycle of operation; If all theharmon-v ics are filtered out of Eg, the galvanometer cura rent is givensimply by Equation 2, in which a: is

shown that the angle timate of the order of magnitude of these sourcesof error, and, what is most important from a practical viewpoint, noattempt has been made to devise a method which makes it possible toeliminate the influence of the finite values oi the quantities A and C,on the characteristics which it is desired to measure.

In order to point out more clearly the nature of my invention, it maybecharacterized as consisting in transforming the known phase sensitivegalvanometer circuit arrangement into a reliable precision measuringcircuit fbr measuring the phase relations, and the correct methods ofdetermining impedances and phase difierences.

In accordance with known practice the bridge circuit of Fig. 2, has itsterminals C and D di-- rectly connected to the grid input terminals oftube T1, 1. e. terminals l and 2 (Fig. 1), 50 as to deliver to it thegrid voltage E whose phase angle with the feiiective plate voltage hasbeen designated by a in Equations 1 and 2. Hereto-- iore the inputterminals A and B of the bridge have been connected directly to avoltage supply Eb, which is either to be compared in phase to the platevoltage supply Ep of.Fig. 1, or which i derived from the same voltagesource Ep and used to determine its frequency, or for the purpose offinding the value of a reactance Z placed into branch BC 01. the bridgeof Fig. 2 by proper adjustment 01' a variable resistance R in branchAC'ot th'e same'bridge.

With this known arrangement let there be an unknown phase angle 0between Eb and Ep, and

is related to the impedance Z and the resistance setting Ru on thevariableresistor R, by the equation wherein X is the effective seriesreactance of Z, its eifective series resistance being 1', wherein 01 Iconsider first the problem of finding the phase phase angle a (i. e.

' bridge of Fig. 2 for example by opening is the phase 'angle Eu withreference to an arbitrary .flxedzero phase. Similarly, the angle then becalculated from Equation 4 above and .from thefollowing equation: 4

r+Ra +x Er=+2rRk+Rk+x==RmRwf5) Ru and Rmare the values to which theresistance R had to beadjusted in the lastitwo measurements withreversed switch positions, in order Inthis equation 02 is the phaseangle of En against the above-mentioned arbitrary fixed zero phase, andRe isythe'value of the variable resistance R which must be set in orderto again obtain the zero galvanometer reading. The 0162) between Em andEbz can thus be found from two null galvanometer readings by simplysubtracting the arc tans of the tangents whose values are given by therighthand sides of Equations 3 and 3'. This is dueto the fact that for anull reading of the gal-v vanometer, the angle must have a definitevalue, viz., the value which makes the left-hand side of Equation 2equal to zero. I

In other words,,the .use'of the combination.

of Figs. 1 and 2has been based on the assumption (which is not generallytrue) that a=90 will make Iga=0 in Equation 2. In accordance with thisinvent on, the phase measurement is now based on Equation 2 quiteindependently of the values whichA and B may have under the'particularconditions of the test. In particular, this phase measurement isindependent. of the tube characteristics and of the stray reactances ofthe transformers and other accidental circuit elements.

Next consider the problem of judging the absolute value of the impedanceA=riiX from null galvanomete'r readings, and the value of R5 to whichthe variable resistor R of branch ACin Fig. 2 has to be adjusted forobtaining this zero reading. It can be shown that the absolute value ofZ is found from two settings R8 and Raz of variable resistor'R,obtained. for direct and reversed connection of voltage Eb of unknownphase angle 0 with respectto Ep', from the equation:

to obtain zero galvanometer reading.)

The sensitivity obtainable is largely determined by the tubes, thegalvanometer, and the. other. constants of the circuit, as well as themagnitudes of Eb and E T1 should have as large a ,transconductance aspracticable.- Any double diode or two tubes operating as single diodesmay be used for T2, the choice depending upon the amount of poweravailable forE and upon convenience R. is chosen to keep the currentwithin therated values for. T1 and T2 and for maximum galvanometer.sensitivity. The galvanometer should be of the zero-center type andshould have a sufficiently high moment of inertia to prevent its pointerfrom showing'vibration at the operating frequency.

In an experimental set-up using a type 6J5 tube for T1, a type 2525 forT2, a galvanometer capable of indicating. current difierences of about1.5 A. R1=R2=5000 ohms, with C=0.500 f, and working at a frequency of 60C. P. S.,Eb being'6 volts R. M. S., and Ep 220 volts R. M. S., theadjustment of R 'was found to be critical to within one ohm, or one partin5000.' When Eb was taken from a heavy duty filament winding on thesame trans former supplying Ep, (0- 5) was found to be 1506 seconds,being (a-r/Z). This shows that the Y constants A and C in Equation 2.are, as might be expected, small compared with B. T

In using the electronic phase bridge as described above, one encountersthree sources of error not met with in ordinary bridge work:

40 (1) Errors caused by currenttaken from the According to the prioruse-oi the combination of Figs. 1 and 2, it was taken for granted thatRel-:RaZ. This is only true, however, under certain conditions which areusually not fulfilled in making practical, measurements. These'conditions are connected substantially with the fact that thequantities A and C (Equation 2) are usually not equal to zero, and thatit is frequentlyvery dimcult actually to determine their relative valuesas comparedwith the quantity B of Equa tion 2.

Equation 4 gives only the absolute value of impedance Z. According to,the invention, the bridge can also be used for determining the values ofand Z of the ohmic and reactive components known ohmicresistance R: isadded in series with impedance Z between circuiting switch M. The valuesof r and X may terminals Band C of the theshort- 'put impedance in the'which can be inserted and removed by bridge by the detector. I v

- (2) Errors resulting .from' .a change in the values of A, B, and C inEquation 2 caused by differences in the peak value of Eg for the twodif-" ferent balance adjustments, and

" (3) Errors caused by the efiects of harmonics if]. Eg.

Fig. 3 showshow the prior arrangement com-. prising the combination ofFigs. 1 and 2 can be modified to-ov'ercome the above-noted sources oferror. In thi arrangement, the voltages Eb whose phases are to becompared with Ep instead of being directly applied to the terminals Aand B, are applied through attenuator 23,- harmonic filter 24.- andphase shifter l8 which can be switched into and out of circuit by adouble pole-double throw switch S3 and whose polarity connection canbereversed by switch S2. Furthermore, the .gridcathode circuit of tubeT1, is provided with an in-' form of a resistance l5 switch $1. One ofthe reasons for using .the phase shifter I8 is that when it is insertedand the bridge balanc'ed before the measurement, it is merely necessarythereafter when using the bridge to measure an unknown impedance Z, totake two readings of.

the resistance R. Furthermore, by means of the adjustable phase shifter,it i possible to adjust thebridgeso that 0=,' and under this conditionIt and R1 willlbe equal and the position of ,switch s: has no efiectonthe bridge balance adjustment.

In that-event, Z will simply be equal to R. i

When using the arrangement to measure frequencies, a known'impedance Zis'inserted and the switch S4 is closed on to its left-hand con Imeasurements is o is reached, for direct and' revers Icharacterlstics'of thistube.

' is fed into input or other circuit elemen same functions-in making theinput impedance of the detector. very high compared witlrthat of thebridge, e. 3., disconnecting the resistor which may be connected betweenthe grid and cathodebf Tr when making the final balance adjustments.Switch 31 in the diagram of Fig. 3 is provided for this purpose. v Tlshould bebiased in anywell-known manner so that its grid is alwaysnegative with respect to its cathode in order to avoid errors from thissource. .Sin'iple cathode bias doesnot fulfill this condition,-

since i drops to zero twice every cycle.

large. It can be removed byadjusting E1." (providingthe adjustment doesnot change its phase) so that E; has always. the same value when finalbalance adjustments are made. A high impedance voltmeter VM (Fig. 3),preferably electronic,xgo

across the input td'Tl will provide for this correc- .tion.- A betterway of.removing this source of error is to make 0 equal to by means pf aphase shifting ;device.' when this condition is obtained R. and R" willbe equal and the position of S: (Fig.

3)- will have no eilect on the bal v ance adjustments. Inthls case, Zwill be simply equal to R.

In practice, the use of th'e phase sliiftershown in His. 3 is alwayindicated if a large number of be made voltage at input #1. It will takebout three to four trial settings of thephase shi ter before thecondition of R= R for zero'galvanometer reading) ednpolarity of switchS2. When usin measuring the absolute impedance Z in the bridge of Fig.3, it is, of course, more expedient toomit tl're-adiustn-ients of thephase shifter of Fig. 3. as in.this case Z may bedetermined from twozero galvanometer readings,- with direct'and reversed polarity oiEtIhe'voltage fed to the input of the bridge ofFlg. 2. For this purpose,switch S3 isclosed on to contacts I! and 20, and switch S4 is closed onto contacts 2! 22. The attenuator. indicated in Fig. 3 serves, for

.ad usting the amplitude oi'the grid input signal to a value at whichthe plate current of tube T1 is kept within practical limi consideringthe The third source of error, namely the harmonics in the detectorinput,-is likely to be troublesome. 1 The relationship between a1, a2,a3, etc., in Equa-.'

tion lnwillvary' according to the setting of R of,

Fig. .2. Hence, in order to obtainaccurate balance indications theharmonics in Eb. or Er, must be filtered out so as not to afl'ectthe-galvanome' ter deflection appreciably. The filter 24 indicated-I inFig. 3 is for the purpose of overcoming the third .source of error abovenoted.

,For making measurements of impedance frequency the signal used 15166.into 'lnput For making measurements of phase relations ps,. power at thefrequencrof the signal under study It is to be understp d that withiiithe spirit and 1 scene. of this invention, other bridge-typecircuits tsmay be .used to serve the are served by the bridge, arrangement shown inthrough an amplifieripreferably or the balanced with a' given input gthe arrangement formin value of a single unknown 2 .and the signalswhose phase rellationships are to be measured are fedrint'o input thedevice herein describedas- 'an'amplifier may be used be-- 1 type) forimproving sensitivity. In addition the filter, tvhen needed, may beplaced either in the iriput or'the. output from the bridge. The expres-5 sion "cosine galvanon'ieter" a employed in the claims is intended tomean a galvanometer arrangement wherein theindications of thegalvanometer are proportional to the cosine of the phase angle beingmeasured.

What is claimed p 1. In a measuring system of the character described,an impedance bridge of the Wheatstone type, a, phase sensitivedetector-arrangement of two conjugate points of said'bridge, means toimpress test voltages across the other conjugate points, thelast-mentioned means including an adjustable phase shifter and areversing switch for J changing the relative phase relations betweensaid means including said phase shifter to balance the Brid e wherebythe zero setting is independent of 'the polarity setting of said switch,said cosine galthetest voltages on. saidother conjugate p'oints eitherdirectly or'through said phase shifter. 5 3. A measuring systemaccording. to claim 1 in grid-controlled electron tube havingits inputter-I alsconnected across said flrst-mentionedconjugate points and itsplate-cathode circuit including a pair of.symmetr ical paths each pathcom-' prising a .potential source, a diode rectlfienand a resistor,whereby said diodes are rendered alto ternately conductive, andagalvanometer connected across said resistors and responsive t the cosineof the phasedifierence between the voltages from said/potential sourcplied to the grid of said tube.

' scribed; an impedance bridge ofthe Wheatstone type, two ratio armsgo'fwhichrare constituted of fixed resistors, the other twoarms of which are1 constituted respectively of a variable resistance and an impedancea'secondbridge of the Wheatstone type;'two ratio arms (n which'areconstituted respectively, of a resistor and a diode with a seriesalternating potential, the other ratio arms of which are likewiseconstituted respectively of 55 a resistor-and a diodewith a series A. 0.potential so that both diodes are conductive in balanced op-. positsphase relations from said a. e; potential, .and a grid-controlledelectron tube having its plate-cathode return connected across one pairof so conjugate points of'said second bridge and its rid-cathode ter'connecte Tpdints of s 5 press a test voltage across the other conjupointspf the. first bridge, the last-mentioned means including areversing switch anda phase shifter arranged tobe adjusted with respectto the I ratio armsof t "t 8pf said pogltion of said switch.

ill

'- which: awacuum-dube voltmeter is connected acros the grid-cathodereturn of said tube to.

eliminate-errors which tend to result from diner;

- the cosine galvanoineter type connected across A The second source oferror is not likely to be 15 other conjugate points and-said phaseshifter; and

claim i m I which another switch is provided for-impressing which"said'phase sensitive detector comprises a" e and thevoltages -ap-.

.4. In a measuring system of the character,de-

return connected acros one-pair of conjugate points of said firstbridge, a g'alva'nomed across-the other-pair ofconjugate aid secondbridge, and means to ime first bridge sothat' the zero set-. t. bridge.is unchanged in. either A measuring system according as claim! inversing switch, and the phase shifter being arence in the peak values ofthe voltages applied to the grid of said tube.

6. A measuring system according to claim 4 in which the saidlast-mentioned means also includes a filter for filtering-out harmonicsof the'said test voltage.

7. In a'measuring system of the character described, a Wheatstonebridge, said bridge having a pair of ratio arms each of which isconstituted of a fixed resistor, the other pair of arms beingrespectively constituted with an adjustable resistor and an impedance;-a phase sensitive detector connected across one pair of conjugatepoints of said bridge, said detector including a grid-controlledelectron tube, a pair of diodes fed in opp site balanced phase relationfrom a source of alternating. voltage, the diodes being connected so asalternately to apply a rectified voltage between the plate and cathodeof said tube, a galvanome ter connected across said diodes and producingan indication which is proportional to the cosine of the phase,difference between the voltages applied to the grid and plate of saidtube; and means to impress a test voltage across the other pair ofconjugate points of said bridge, the last-mentioned means including aphase shifter and a re.

ranged for adjustments so that the zero setting of said bridge isindependent of the polarity setrelative phase relations between saidother conand said phase shifter, and means phase shifter to balance thebridge jugate points including said so that the zero setting isindependent of the polarity setting 01 said switch,'said cosinegalvanometer including a conjugate network having two pairs of conjugatepoints, a grid-controlled amplifier having its plate-to-cathode circuitconnected across one of said pairs of conjugate points -of said networkand an indicating galvanometer the other pair of said conjugateconnected across points of said network.

ROBERT HENRY BROWN.-

the character de-

